Abstracts: A Renaissance in the Analysis of Turbidite Systems?

ELLIOTT, TREVOR

For a long time our ideas on the nature of sedimentation by turbidity currents have been conditioned by a view that the flows are initiated by mass failure on the slope and therefore involve resedimentation of previously deposited material. There is widespread evidence to support this view (e.g., the Grand Banks earthquake), and it leads to a notion of episodic, catastrophic turbidity currents characterized by high magnitude, low frequency, and a high degree of randomness in flow properties. There is, however, an increasing body of evidence from side-scan sonar records of modern fans, three-dimensional seismic data on ancient fans, and exposed turbidite successions that challenges this view and suggests an alternative. Evidence of intensely meandering channel patterns in the side-scan sonar and seismic data sets implies the operation of more steady, uniform turbidity currents that interact in a sensitive manner with the channel morphology. Similarly, exposed turbidite successions often display evidence of numerous flows with similar properties that, once again, interact sensitively with channel geometries. Collectively, these observations suggest that in specific settings turbidity currents may be of small to moderate magnitude, relatively high frequency, and that they may be more ordered and repetitive in their flow characteristics. These points appear to apply most forcibly at sites of delta-driven turbidite sedimentation during periods of sea level lowstand. Under these conditions the frequency of gravity-driven underflows initiated during periods of flood-generated high discharge at river mouths may be considerably in excess of present-day occurrences of this process. High sediment flux and a lack of storage/accommodation space in the coastal plain may contribute to a higher frequency of gravity underflows at the river mouth which, in turn, could lead to the formation of virtually discharge-driven turbidity currents that may account for some of the above observations. This analysis implies that the products of failure-driven/resedimentation turbidites and discharge-driven turbidites may have to be distinguished in the future and supports the notion that many turbidity currents may be more steady than previously envisaged.

AAPG Search and Discovery Article #90938©1997-1998 AAPG Distinguished Lecturers